THE ZEUS CALORIMETER FIRST LEVEL TRIGGER

An overview of the ZEUS calorimeter first level trigger is presented. The CFLT uses a pipelined architecture to accept and analyze calorimet er data for every 96 ns beam crossing interval. PMT signals are combin ed by analog electronics into electromagnetic and hadronic sums for 89 6 trigger towers. The analog sums are then digitized and analyzed. The CFLT determines the total, transverse, and missing transverse energy, identifies isolated electrons and muons, and sums energies in program mable subregions. Calculations are performed in 96 ns steps, and new d ata are accepted for every beam crossing. Trigger data are forwarded t o the global first level trigger (GFLT) after 2 mu s, allowing a GFLT accept to be issued 5 mu s after the beam crossing which produced the event. Important features of the CFLT include a 12-bit effective dynam ic range, extensive use of memory lookup tables for trigger calculatio ns, fast pattern searches for isolated leptons, and low electronics no ise. During the 1993 HERA run, the CFLT reduced a 50 kHz background ra te to around 100 Hz.